U.S. patent application number 14/250872 was filed with the patent office on 2014-08-07 for method of component assembly on a substrate.
This patent application is currently assigned to Newsouth Innovations Pty Limited. The applicant listed for this patent is Newsouth Innovations Pty Limited. Invention is credited to Till Bocking, Michael Gal, Katharina Gaus, John Justin Gooding, Qiao Hong, Kristopher A. Kilian, Peter John Reece.
Application Number | 20140220670 14/250872 |
Document ID | / |
Family ID | 43030678 |
Filed Date | 2014-08-07 |
United States Patent
Application |
20140220670 |
Kind Code |
A1 |
Bocking; Till ; et
al. |
August 7, 2014 |
METHOD OF COMPONENT ASSEMBLY ON A SUBSTRATE
Abstract
A method of component assembly on a substrate, and an assembly
of a bound component on a substrate. The method comprises the steps
of forming a free-standing component having an optical
characteristic; providing a pattern of a first binding species on
the substrate or the free standing component; and forming a bound
component on the substrate through a binding interaction via the
first binding species; wherein the bound component exhibits
substantially the same optical characteristic compared to the
free-standing component.
Inventors: |
Bocking; Till; (Boston,
MA) ; Gooding; John Justin; (Chippendale, AU)
; Kilian; Kristopher A.; (Chicago, IL) ; Gal;
Michael; (Engadine, AU) ; Gaus; Katharina;
(Chippendale, AU) ; Reece; Peter John;
(Abbotsford, AU) ; Hong; Qiao; (Westmead,
AU) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Newsouth Innovations Pty Limited |
New South Wales |
|
AU |
|
|
Assignee: |
Newsouth Innovations Pty
Limited
New South Wales
AU
|
Family ID: |
43030678 |
Appl. No.: |
14/250872 |
Filed: |
April 11, 2014 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12740734 |
Jul 26, 2010 |
8722437 |
|
|
PCT/AU2008/001616 |
Oct 31, 2008 |
|
|
|
14250872 |
|
|
|
|
11933541 |
Nov 1, 2007 |
|
|
|
12740734 |
|
|
|
|
Current U.S.
Class: |
435/287.9 ;
257/13; 372/45.012; 422/69 |
Current CPC
Class: |
G02B 5/0816 20130101;
G01N 21/63 20130101; H01L 33/06 20130101; H01S 5/34 20130101; H01L
33/105 20130101; G01N 33/54373 20130101; B81C 3/002 20130101; G02B
5/285 20130101 |
Class at
Publication: |
435/287.9 ;
257/13; 372/45.012; 422/69 |
International
Class: |
G01N 33/543 20060101
G01N033/543; H01S 5/34 20060101 H01S005/34; H01L 33/10 20060101
H01L033/10; H01L 33/06 20060101 H01L033/06 |
Foreign Application Data
Date |
Code |
Application Number |
May 8, 2008 |
AU |
2008902248 |
Claims
1. A sensor structure comprising: a first Bragg mirror; a second
Bragg mirror that is a free-standing component; and a stimuli
responsive material disposed between the first and second Bragg
mirrors; wherein the second Bragg mirror is assembled on the first
Bragg mirror by a binding interaction via the stimuli responsive
material.
2. The sensor structure of claim 1, wherein the stimuli responsive
material is disposed between the first and second Bragg mirrors
such that infiltration of the stimuli responsive material is
prevented.
3. The sensor structure of claim 2, wherein derivatisation of a
surface of at least one of the first Bragg mirror, the second Bragg
mirror, or both was performed prior to deposition of the stimuli
responsive material.
4. The sensor structure of claim 1, wherein a stimuli for the
stimuli responsive material comprises one or a group of a
biomolecule, a chemical, a temperature, light, a pH, a voltage, or
a mechanical force.
5. The sensor structure of claim 1, wherein the stimuli responsive
material comprises one or more of gelatin, extracellular matrix
biopolymers, proteins, oligosaccharides, proteoglycans, recombinant
polypeptides, synthetic polypeptides, nucleic acids, synthetic
co-polymer systems, small molecule and nano-object encapsulated
polymers, pNIPAM, lipids, carbohydrates, cellulose, cells, plant or
animal tissue, polymers of any type, hydrogels, microorganisms,
nanoparticles, or nanowires.
6. The sensor structure of claim 1, wherein the first Bragg mirror,
the second Bragg mirror, or both are configured to be responsive to
a further stimuli.
7. The sensor structure of claim 1, wherein the first and second
Bragg mirrors comprise PSi nanoporous structures.
8. The sensor structure of claim 1, wherein the first Bragg mirror
is formed on a substrate.
9. The sensor structure of claim 8, wherein the substrate and the
second Bragg mirror are lattice mismatched.
10. The sensor structure of claim 1, wherein the first Bragg mirror
exhibits substantially the same optical characteristic as the
second Bragg mirror.
11. A light emitting device comprising: a first Bragg mirror; a
second Bragg mirror that is a free-standing component; and a light
emitting material disposed between the first and second Bragg
mirrors; wherein the second Bragg mirror is assembled on the first
Bragg mirror by a binding interaction via the light emitting
material.
12. The light emitting device of claim 11, wherein the light
emitting material comprises at least one of a biorecognition
element, a complementary biomolecular species, or quantum dots.
13. The light emitting device of claim 11, wherein the light
emitting material comprises a mixture of at least one
biorecognition element, at least one complementary biomolecular
species, and quantum dots, wherein the binding interaction
comprises the quantum dots being bound via pairs of the at least
one biorecognition element and the at least one complementary
biomolecular species.
14. The light emitting device of claim 13, wherein the second Bragg
mirror is assembled on the first Bragg mirror by the binding
interaction by the at least one biorecognition element, the at
least one complementary biomolecular species, and the quantum dots
diffusing into respective interfacial regions of the first and
second Bragg mirrors
15. The light emitting device of claim 13, wherein the quantum dots
comprise II-VI semiconductor quantum dots or III-V semiconductor
quantum dots.
16. The light emitting device of claim 11, wherein the light
emitting material comprises at least one of different types of
quantum dots in different lateral areas within a layer or different
types of quantum dots in different layers.
17. The light emitting device of claim 11, wherein the light
emitting material further comprises a gain material configured to
facilitate lasing.
18. The light emitting device of claim 11, further comprising an
optical cavity disposed at the interface between the first and
second Bragg mirrors.
19. The light emitting device of claim 18, wherein a thickness of
the optical cavity matches an emission wavelength of the light
emitting material.
20. The light emitting device of claim 11, wherein the first Bragg
mirror is formed on a substrate.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 12/740,734, filed Jul. 26, 2010, which is a national stage
application of PCT/AU2008/001616, filed Oct. 31, 2008, which is a
continuation-in-part of U.S. application Ser. No. 11/933,541, filed
Nov. 1, 2007, hereby incorporated by reference. This application
also claims priority to Australian Application No. 2008902248,
filed May 8, 2008, hereby incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates broadly to a method of
component assembly on a substrate, to an assembly of a bound
component on a substrate, to an sensor structure and a method of
fabricating the same, and to a light emitting device and a method
of fabricating the same.
BACKGROUND
[0003] The creation of integrated optical devices from separate
micro-components has, in the past, required time-consuming and
often manually intensive methods. Attempts to alleviate these
difficulties have seen the emergence of more mechanized
technologies that focus on assembly either via fluidic
self-assembly or methods that are based on wafer-to-wafer transfer.
Key to all these technologies is the substrate which is either a
specifically prepared `receptor` with precisely etched holes that
are complementary to the optical components, or substrates that
require equally stringent photolithographic alignment and/or
masking. The current technologies used for the integration of
optical components are restricted by the limited number of
compatible substrates (e.g. silicon, silicon oxide, gallium
arsenide).
[0004] Ideally, the optical designer should not be limited by the
fabrication technology. For example, one should be able to
integrate III-V light sources and detectors with Si based photonic
crystals, modulators and/or micro-mirrors, with SiO.sub.2
waveguides, and non-linear optical devices on any substrate. The
function and/or complexity of an integrated optical circuit should
not be restricted by the substrate.
[0005] "Strained layer epitaxy" is used to integrate semiconductors
with dissimilar lattice structures, such as growing GaAs on Si, or
SiGe alloys on Si, etc. However, this technique is only possible if
the respective layer thicknesses are thinner than a critical
thickness which is typically extremely thin. In addition, this
technique is only useful for crystalline materials, and is not
useful for integrating non-crystalline materials such as plastics
and glasses. The use of MEMS (Micro-Electro-Mechanical Systems) for
integrating mechanical components, sensors, etc. with electronics
on a silicon substrate using microelectronic technology is also
made use of This technology relies on devices, such as
micro-mirrors, waveguides, cantilevers, etc that are Si (and
SiO.sub.2) based and are micromachined into Si. Again, this method
is limited to Si and SiO.sub.2 and is not useful to integrate other
materials, such as GaAs, electro-optic materials, etc
[0006] There are a number of other techniques that are grouped into
`top-down` and `bottom-up` approaches. The top-down approach
involves a block of material being processed into the desired shape
and working unit. In bottom-up fabrication, small building blocks
(usually nanoscale as the term originates from nanotechnology) are
connected together to fabricate a functioning unit.
[0007] Current top-down approaches for integrating optical
structures on a substrate typically involve fluidic assembly into
defined `holes` in a substrate, lithographic patterning followed by
etching or wafer-to-wafer transfer. These are very complicated
procedures that lack the ability to be easily scaled up and
typically suffer from low fabrication success rates.
[0008] On the other hand, while there are many potential bottom-up
strategies for fabricating optical structures on different
materials, no current method for assembling high quality optical
devices (prefabricated) on any substrate has been demonstrated. A
sufficient understanding of how to assemble molecular building
blocks with sufficient control to produce high quality materials
(that is, comparable to microelectronics state of the art) has not
been reached.
[0009] Recently, methods for electric field assisted self-assembly
of functionalized DNA strands as building blocks for assembly and
fabrication of devices have been proposed in U.S. Pat. No.
6,652,808. However, the methods disclosed in that document focus
primarily on the control and chemical nature of the DNA based
building blocks for bonding of components to a substrate, rather
than providing any teaching with respect to the properties or
functionality of the devices bound to the substrate. Furthermore,
an approach for building a photonic band-gap structure is
disclosed, where a photonic band-gap structure is built-up from
metal beads exhibiting magnetic properties. The photonic band-gap
structure is formed on the substrate through a process in which the
metal beads are interconnected via DNA bonds. No optical
characterization of such grown photonic band-gap structures is
provided in that document.
[0010] Furthermore, there is no teaching provided in that document
that verifies whether the alignment accuracy between the metal
beads is actually sufficient to achieve a photonic crystal effect,
and on which substrate or type of substrates. A technique for
alignment of "larger" structures of the order of 10 to 100 microns
is also discussed in that document, using selective derivatisation
with different DNA sequences of a device to be positioned and
oriented on a substrate. However, no teaching is provided with
respect to handling of larger devices, thus limiting the proposed
method to techniques in which the devices to be attached are
smaller than about 100 microns, and with a need to apply individual
devices in that size range to the substrate for assembly. The
preparation of free-standing devices in that range of small sizes
can constitute a major challenge in the overall assembly process,
in particular with a view to mass-production of assemblies of
devices on various substrates.
[0011] As an example application of integrated optical devices,
currently, optical methods for sensing molecular species often
require a sample cleanup, where the target analyte resides in a
complex mixture of many different molecules. Many current optical
methods also require the labeling of the analyte using for example,
a fluorescent tag, and complex instrumentation that requires both
transport of the sample to a laboratory and trained personnel. The
prior art optical methods also require time-consuming protocols
with long incubation periods, wash steps etc. The combination of
these factors will often lead to the slow detection of a chemical
or a biological molecule. However, in many situations, expediency
is integral in detecting a substance for example, at times of
environmental threat, point-of-care diagnosis, biological and
chemical warfare. Hence, many prior art sensing technologies are
inadequate. Although there are currently a number of label-free
methods for sensing molecular species, these methods suffer from
either non-specific detection issues, poor sensitivity compared to
labeling approaches, incompatible formats for the field or other
disadvantages such as complicated instrumentation, the need for
skilled technicians or the need for sample cleanup or a combination
of the above.
[0012] Photonic crystals formed by electrochemical etching porous
silicon (PSi) are an example of `hard` photonic crystals that can
be fabricated by modulating the porosity and hence the refractive
index of the layers during anodization [A. G. Cullis, L. T. Canham,
P. D. J. Calcott, Applied Physics Reviews 1997, 82, 909.] The
nanoporous architecture of the PSi material allows infiltration of
gases and liquids within the material, thus modifying the average
refractive index and the resultant spectral qualities. This quality
of PSi materials has led to numerous investigations of PSi
materials in optical sensing including gas, chemical and biological
sensing. [M. P. Stewart, J. M. Buriak, Adv. Mater. (Weinheim, Ger.)
FIELD Full Journal Title: Advanced Materials (Weinheim, Germany)
2000, 12, 859.; S. D'Auria, M. de Champdore, V. Aurilia, A.
Parracino, M. Staiano, A. Vitale, M. Rossi, I. Rea, L. Rotiroti, A.
M. Rossi, S. Borini, I. Rendina, L. De Stefano, J. Phys.: Condens.
Matter FIELD Full Journal Title: Journal of Physics: Condensed
Matter 2006, 18, S2019.; G. Marsh, Mater. Today (Oxford, U. K.)
FIELD Full Journal Title: Materials Today (Oxford, United Kingdom)
2002, 5, 36.; T. Islam, H. Saha, Sens. Actuators, A FIELD Full
Journal Title: Sensors and Actuators, A: Physical 2007, A133,
472.]
[0013] One type of PSi photonic crystal that has shown utility for
sensing is the resonant microcavity. [P. J. Reece, M. Gal, H. H.
Tan, C. Jagadish, Applied Physics Letters 2004, 85, 3363.; L.
Rotiroti, L. D. Stefano, I. Rendina, L. Moretti, A. M. Rossi, A.
Piccolo, Biosensors & Bioelectronics 2005, 20, 2136.; L. D.
Stefano, I. Rea, I. Rendina, L. Rotiroti, M. Rossi, S. D'Auria,
Physica Status Solidi A: Applications and Materials Science 2006,
203, 886.; L. A. DeLouise, B. L. Miller, Analytical Chemistry 2004,
76, 6915.; L. A. DeLouise, B. L. Miller, Analytical Chemistry 2005,
77, 1950.; L. A. DeLouise, P. M. Kou, B. L. Miller, Analytical
Chemistry 2005, 77, 3222.; H. Ouyang, M. Christophersen, R. Viard,
B. L. Miller, P. M. Fauchet, Advanced Functional Materials 2005,
15, 1851.; H. Ouyang, L. A. DeLouise, B. L. Miller, P. M. Fauchet,
Analytical Chemistry 2007, 79, 1502.; H. Ouyang, C. C. Striemer, P.
M. Fauchet, Applied Physics Letters 2006, 88, 163108.].
Microcavities are formed by incorporating a defect (spacer) layer
within the periodicity of a multilayered 1-dimensional photonic
crystal stack. Tuning the optical thickness (n d, where n is the
refractive index and d the thickness of the layer) of the spacer
layer to m.lamda./2 (.lamda. is the central wavelength of the Bragg
plateau, m is the spectral order) gives rise to a cavity resonance
in the centre of the spectrum, where light of that wavelength
"resonates" and therefore does not reflect.
[0014] In the prior arts using PSi microcavities for sensing
stimuli such as biomolecules or chemicals etc., the infiltration of
material can cause shifts in the entire spectrum that can be
correlated to the influx of material throughout the nanoporous
matrix. Another drawback to using microcavities for sensing in
existing sensor designs associated with the requirement that
stimuli must reach the central layer is that the stimuli will need
to penetrate from the top layer of the micro cavity through the
nanoporous architecture, a particular problem for large
biomolecules (comparable to or larger than the smallest pore size
in the alternating pore size multi layered stack pore size).
Attempts to alleviate this problem have included enlarging the pore
diameter which leads to decreased optical quality and sensitivity.
[H. Ouyang, C. C. Striemer, P. M. Fauchet, Applied Physics Letters
2006, 88, 163108.] Other attempts to address this problem have
included modifying the surface chemistry within the nanoporous
matrix which may enhance the ingress of particular species, the
diffusion issue is still not solved. Hence, the modification of
surface chemistry may allow excellent control over the type of
analyte captured but its use is still limited by the diffusion
issue.
[0015] As another example application of integrated optical
devices, currently, there is a research interest into fabricating
Si integrated optical epitaxial light emitting structures for
optoelectronic technologies. While II-VI quantum dot doped
microcavities have been reported for TiO.sub.2--SiO.sub.2
distributed Bragg reflectors have been reported e.g. in [L Guo, T D
Krauss, C B Poitras, M Lipson, X Teng and H Yang, Applied Physics
Letters 89, 061104 (2006)], and ion doped porous Si microcavities
e.g. in [H A Lopez and P M Fauchet, Applied Physics Letters 77,
number 23, 4 Dec. 2000], the applicant is not aware of reports on
quantum dot doped microcavities formed using Si integrated optical
epitaxial techniques.
[0016] The present invention has been made in view of the above
described background to seek to address one or more of the
above-mentioned problems.
SUMMARY
[0017] In accordance with a first aspect of the present invention
there is provided a method of component assembly on a substrate,
the method comprising the steps of forming a free-standing
component having an optical characteristic; providing a pattern of
a first binding species on the substrate or the free standing
component; and forming a bound component on the substrate through a
binding interaction via the first binding species; wherein the
bound component exhibits substantially the same optical
characteristic compared to the free-standing component.
[0018] In accordance with a second aspect of the present invention
there is provided a assembly comprising a substrate; and a bound
component assembled on the substrate through a binding interaction
via a first binding species provided on the substrate or on a
free-standing pre-form of the bound component; wherein the bound
component exhibits substantially a same optical characteristic
compared to the free-standing pre-form.
[0019] In accordance with a third aspect of the present invention
there is provided a sensor structure comprising a first Bragg
mirror; a second Bragg mirror; and a stimuli responsive material
disposed between the first and second Bragg mirrors; wherein the
second Bragg mirror is assembled on the first Bragg mirror by a
binding interaction via the stimuli responsive material.
[0020] In accordance with a fourth aspect of the present invention
there is provided a method for fabricating a sensor structure, the
method comprising the steps of providing a first Bragg mirror;
providing a second Bragg mirror; and providing a stimuli responsive
material disposed between the first and second Bragg mirrors;
wherein the second Bragg mirror is assembled on the first Bragg
mirror by a binding interaction via the stimuli responsive
material.
[0021] In accordance with a fifth aspect of the present invention
there is provided a method of fabricating a light emitting device,
the method comprising the steps of providing a first Bragg mirror;
providing a second Bragg mirror; and providing a light emitting
material disposed at an interface between the first and second
Bragg mirrors; wherein the second Bragg mirror is assembled on the
first Bragg mirror by a binding interaction via the light emitting
material.
[0022] In accordance with a sixth aspect of the present invention
there is provided a light emitting device comprising a first Bragg
mirror; a second Bragg mirror; and a light emitting material
disposed at an interface between the first and second Bragg
mirrors; wherein the second Bragg mirror is assembled on the first
Bragg mirror by a binding interaction via the light emitting
material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Embodiments of the invention will be better understood and
readily apparent to one of ordinary skill in the art from the
following written description, by way of example only, and in
conjunction with the drawings, in which:
[0024] FIG. 1 shows a schematic representation of assembly of
optical components according to an example embodiment.
[0025] FIGS. 2a-d show the characteristic optical reflectivity
spectra of a PSi microcavity as prepared, and assembled on GaAs,
silicon dioxide and poly carbonate respectively, using the method
of FIG. 1.
[0026] FIG. 3a shows reflectivity spectra of two different
microcavities assembled on the same polycarbonate substrate using
the method of FIG. 3b.
[0027] FIG. 3b shows a schematic representation of attachment of
two different microcavities onto different locations of the same
substrate according to an example embodiment.
[0028] FIG. 4 shows a schematic representation of the assembly of
microcavities from parts according to an example embodiment.
[0029] FIGS. 5a and b show reflectivity spectra of structures
fabricated using the method of FIG. 4 before and after assembly of
mirrors.
[0030] FIGS. 6a to c show reflectivity spectra of a Bragg mirror
and different assembled microcavity structures fabricated using the
method of FIG. 4.
[0031] FIG. 7 shows a scanning electron microscopy (SEM) image of a
structure fabricated using the method of FIG. 4.
[0032] FIG. 8 shows a profilometry trace of the structure of FIG.
7.
[0033] FIG. 9 shows details of the success rate of assembling a
final microcavity using the method of FIG. 4.
[0034] FIG. 10 is a schematic representation showing assembly of
microcavities on a substrate using a sandwich approach according to
another embodiment.
[0035] FIGS. 11a to d show the optical properties of a substrate
reflector and formed microcavities with different spacer layers
respectively fabricated using the method of FIG. 10.
[0036] FIG. 12a shows reflectivity spectra of a PSi Bragg mirror
before and after deposition of a PMMA layer by spin coating,
according to another example embodiment.
[0037] FIG. 12b shows reflectivity spectra of microcavities
fabricated using a PMMA spacer layer in the method of FIG. 10.
[0038] FIG. 13 shows a flow chart illustrating a method of
component assembly on a substrate according to an example
embodiment.
[0039] FIGS. 14a-d show schematic cross-sectional drawings
illustrating fabrication of a sensor structure according to an
embodiment of the present invention.
[0040] FIGS. 15a and b show schematic cross-sectional drawings
illustrating the coating of a spacer material onto a surface of a
Bragg mirror according to an embodiment of the present
invention.
[0041] FIGS. 16a-e show photographs of the different stages in the
formation of a sensor structure and a scanning electron microscope
image of the sensor structure according to an embodiment of the
present invention.
[0042] FIGS. 17a-d show graphs illustrating optical reflectance
spectra of sensor structures according to different embodiments of
the present invention.
[0043] FIG. 18 shows an experimental setup for a sensing
application of a sensor structure according to an embodiment of the
present invention.
[0044] FIGS. 19a-c show experimental results using the experimental
setup in FIG. 18 according to an embodiment of the present
invention.
[0045] FIGS. 20a and b show further experimental results using the
experimental setup in FIG. 18 according to an embodiment of the
present invention.
[0046] FIGS. 21a-b show the optical reflectance spectra before and
after proteolysis occurs in a sensor structure according to an
embodiment of the present invention.
[0047] FIG. 22 shows a plot illustrating the shift in the optical
spectrum after exposure of a sensor structure to water vapour
according to an embodiment of the present invention.
[0048] FIG. 23 shows a flow chart illustrating a method of
fabricating a sensor structure according to an embodiment of the
present invention.
[0049] FIG. 24 shows a schematic cross-sectional view of a light
emitting device according to an example embodiment.
[0050] FIG. 25 shows a scanning electron microscopy (SEM) image of
the fabricated structure in an example embodiment.
[0051] FIG. 26 shows reflectance spectra measured for different
stages of the fabrication of the light emitting device of the
example embodiment.
[0052] FIG. 27 shows the measured photo luminescence of the light
emitting device of the example embodiment.
[0053] FIG. 28 shows a comparison of the photo luminescence
measured for the example light emitting device of the example
embodiment, with a photo luminescence measurement for the same QDs
deposited on silicon using the same fabrication times.
[0054] FIG. 29 shows a plot of photo luminescence intensity versus
excitation intensity from 1 .mu.W to 5 mW of the light emitting
device of the example embodiment.
[0055] FIG. 30 shows a plot of photo luminescence intensity versus
excitation intensity from 10 mW to 1 .mu.W of the light emitting
device of the example embodiment.
[0056] FIG. 31 shows photo luminescence intensity versus incubation
time graphs for different streptavidin-QD incubation times of 10
minutes for the light emitting device of the example
embodiment.
[0057] FIG. 32 shows a flow chart illustrating a method of
fabricating a light emitting device according to an example
embodiment.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0058] The integration of different optical components on the same
substrate, as well as optical components with electronic devices,
has been hindered by different components typically being made of
different materials. Hence a problem has existed where either
optical components are all made from the same material, hence
compromising the performance of some or all of the components, or
the problem has been how to integrate components made from the
different materials onto the same substrate. Thus the problem is
one of material incompatibility. The described example embodiments
provide methods that can overcome this problem by harnessing the
recognition properties of biological molecules to enable the
assembly of optical materials on any substrate. Porous silicon
(PSi) microcavities and Bragg mirrors are fabricated and assembled
on silicon, gallium arsenide and plastic. The substrate material is
modified by application of a biological molecule to define the
location for assembly. Optical components modified with the
complementary biomolecule self-assemble only onto the correct
location without compromising their optical integrity. In another
embodiment optical components can be deposited onto and adhered to
a substrate via patterns of an adhesive ultrathin coating.
Furthermore, the technique in the example embodiments allows
assembly of new devices from components of different composition as
demonstrated by incorporating different spacer layers between
porous silicon Bragg mirrors to create a resonant microcavity.
[0059] Described embodiments use biomolecule directed or adhesive
coating directed assembly of prefabricated high quality optical
structures on the micro and macroscale without micromachining
requirements. In contrast to biomolecule directed assembly of
photonic crystals from colloidal building blocks (described e.g. in
U.S. Pat. No. 6,752,868 B2), which cannot produce the high quality
optical structures required for the fabrication of optical
circuits, in example embodiments high quality Bragg mirrors and
resonant microcavities were formed by anodization of silicon. In
one embodiment, the macroscale assembly of optical films occurs on
substrates patterned with complementary biological molecules. The
high affinity of biorecognition causes assembly at the applied
pattern only, while the remainder of the film fractures upon
rinsing and drying steps leaving a macroscale pattern of optical
structures (>1 mm) In another embodiment, a macroscopic
free-standing optical structure was fractured by sonication in
ethanol to produce microparticles (<100 .mu.m). Utilizing
biorecognition, the optical microparticles are assembled in the
correct orientation when applied to the biomolecule labelled
substrate. Example embodiments of the present invention can create
optically flat materials on a macroscale such that high quality
optical characteristics are maintained. In contrast to building an
optical structure using the bottom up approach, example embodiments
can allow assembly of prefabricated high quality optical components
over multiple length scales.
[0060] Example embodiments assemble optical materials on any
substrate that allows biorecognition or deposition of thin coatings
to mate the materials together. In one embodiment, resonant
microcavities fabricated with porous silicon were removed from
silicon and coated with biorecognition molecules. A number of
substrates including: silicon, silicon dioxide, galium arsenide and
polycarbonate, were patterned with aqueous solutions of
complementary biomolecules. Application of the labelled
microcavities to the patterned substrates yielded assembly at the
biomolecular pattern only, while the remaining microcavity was
rinsed away with ethanol.
[0061] Example embodiments provide a combination of high quality
top-down optical structure fabrication techniques with a bottom-up
assembly method (a hybrid approach) exploiting biorecognition or an
adhesive coating to form new devices. Previous work on assembling
optical structures has involved either 1) the top-down fabrication
of optical materials (e.g. PSi microcavity formation) or 2)
bottom-up assembly of new optical materials (e.g. colloidal crystal
fabrication). By first forming high quality optical materials using
top-down fabrication followed by e.g. biomolecule directed assembly
of multiple components, a high quality optical structure can be
created in example embodiments. Other materials (e.g. responsive
polymers and small molecules, metals, nanoparticles and objects,
redox and photosynthetic proteins, molecular wires, carbon
nanotubes, ionic liquids/liquid crystals, lipid layers, cells,
diatoms, silica and polymer beads and many other functional
molecules and materials) can be incorporated with the high quality
optical structures such that novel properties and new emergent
functions may be harnessed.
[0062] FIG. 1 shows a schematic representation of the assembly of
optical components by specific adhesion onto any substrate via
biomolecular interactions in an example embodiment. Porous Silicon
(Psi) optical resonant microcavities (1D photonic crystals) are
prepared as free-standing films 100c in a first sequence and then
deposited via biorecognition-mediated self-assembly onto a
substrate 154 in a second sequence. The photographs in FIG. 1 show
top views of an as prepared PSi Bragg mirror 100a and the PSi Bragg
mirror 100b after application of a current pulse. The PSi film 100b
remains attached to the wafer 104 around the edge allowing
modification with proteins on the top surface 102 while the bottom
surface remains unmodified. It is noted that the components are not
drawn to scale; the thickness of the free-standing PSi photonic
crystal 100c is between 1.5-3 .mu.m whereas the thickness of the
combined ligand and receptor layer 152 is in the order of 10 nm.
The assembly of microcavities with spacer layers of optical
thickness corresponding to the half wavelength of visible light (n
d=.lamda./2) in the example embodiment demonstrates the capability
to assemble delicate optical devices that can be tested and
characterized. PSi has proven to be particularly well-suited for
the production of high quality optical devices, such as
one-dimensional photonic crystals including Bragg mirrors, optical
filters and microcavities, as its refractive index can be precisely
and continuously tuned between approximately 1.3 and 3.0. The PSi
based microcavities are fabricated by electrochemical etching the
single crystal Si wafer 104, whereby the etching-current density
determines the porosity and hence the refractive index of the
material.
[0063] For the PSi film 100a photonic crystal formation, the
Si(100) wafer 104 (p++, B-doped, 0.005 .OMEGA. ohm cm, single side
polished) was cleaned by sonication in ethanol and acetone and
blown dry under a stream of nitrogen. The cleaned wafer 104 was
etched in an electrochemical cell with a polished stainless steel
electrode as back-contact and a Pt ring counter electrode using 25%
ethanolic HF (mixture of 50% aqueous HF and 100% ethanol, 1:1, v/v)
as electrolyte. The power supply was controlled using custom
written software to modulate the current density and etching times
during the etching process. Etch stops were incorporated into the
etching program to allow recovery of the HF concentration at the
etching front. The current densities and etch times required to
obtain the PSi layer 100a of desired porosity and thickness were
calculated from calibration curves obtained for each batch of Si
wafers and etching solutions.
[0064] At the end of the electrochemical etching that creates the
cavity, a high current pulse is applied (FIG. 1, Step a) to
lift-off most of the microcavity from the underlying Si wafer 104.
As a result, the approximately 3 .mu.m thick PSi film 100b
(microcavity), in this example, becomes free from the underlying
substrate but remains attached at the edges. Maintaining the cavity
attached to the Si wafer 104 is advantageous to enable simple
further modifications for the self-assembly process. For details of
a suitable technique to achieve "lift-off" reference is made to [H.
Koyama, M. Araki, Y. Yamamoto, N. Koshida, Japanese Journal of
Applied Physics 30, 3606 (1991)], the contents of which are hereby
incorporated by cross reference. After lift-off, the sample was
carefully rinsed with ethanol followed by pentane and dried under a
very gentle stream of nitrogen with gentle heating. The
modification employed in this example involves the physisorption of
a particular biorecognition element (e.g. a ligand) onto the
exposed surface 102 of the microcavity 100b (FIG. 1, Step b).
Proteins (e.g. avidin or biotinylated albumin) were deposited onto
the hydrophobic surface of as-prepared PSi film 100b by
physisorption from aqueous solution. Aqueous solutions do not enter
the pores of as-prepared PSi film 100b.
[0065] Subsequently, the modified device 100c is released from the
Si wafer 104 (FIG. 1, Step c) and inverted onto a substrate 154 of
choice which is pre-modified with a pattern of the complementary
biomolecular species 156 (e.g. a receptor) (FIG. 1, Step d). The
protein-modified lift-off sample 100b (still attached at its edge
to the underlying Si wafer 104) was released from the Si wafer 104
by scoring the edge of the PSi film 100b with a sharp tip and
floating the released PSi film 100c off the Si wafer 104 in this
example embodiment. The assembly substrate 154 was spotted with
solutions of protein to define the positions for adhesion.
Subsequently, poly(ethylene glycol) was physisorbed elsewhere onto
the substrate surface as a blocking species in this example
embodiment to diminish binding of the protein-modified
free-standing Psi film 100c to the bare substrate 154 surface.
Portions 158, 160 of the PSi photonic crystal 100c not bound to the
substrate 154 via the biorecognition pair 152 can simply be washed
away to leave microcavities 100d only bound at positions determined
by the receptor pattern 156 on the substrate 154 (FIG. 1, Step e).
The substrate 154 was then vigorously rinsed to remove non-bound or
weakly bound portions 158, 160 of the PSi film 100c elsewhere on
the substrate 154. Removal of avidin-modified portions 158, 160
non-specifically adhering to the BSA-coated substrate 154 areas was
performed using a detergent in the removal process in the example
embodiment. Depending on the nature of the binding species in
different embodiments, the use of a detergent is optional.
[0066] It is noted that other blocking species may be used in
different embodiment, including, but not limited to, thin films of
or self assembled monolayers (SAMs) terminated with [0067] ethers
and derivatives of poly-/oligo-(ethylene glycol) [0068]
amines/ammonium salts [0069] amides, amino acids, peptides [0070]
Crown ethers [0071] sugars, polyols (eg mannitol) [0072]
surfactants (eg Triton X-100) [0073] zwitterionic groups (eg
phosphrylcholine) [0074] perfluorinated groups [0075] protein
[0076] synthetic polymers [0077] natural polymers or combinations
thereof. It is noted that, depending on the nature of the binding
species in different embodiments, the use of a blocking species is
optional.
[0078] As seen in FIG. 1, the method in the example embodiment
results in an assembly comprising the substrate 154 and the bound
microcavities 100d assembled on the substrate 154 through a binding
interaction via a binding species in the form of a biorecognition
pair. In another embodiment described below, the binding species
can be in the form of an adhesive layer provided on the substrate
or the free-standing component.
[0079] It is important to note that the optical properties of the
devices advantageously remain the same independent of the substrate
in different example embodiments. FIGS. 2a-d show the
characteristic optical reflectivity spectra 200 to 203 of the same
PSi microcavity (compare 100d in FIG. 1) as prepared (before
lift-off), and assembled on GaAs, silicon dioxide and
polycarbonate, respectively, as directed by the interaction between
the protein avidin on the device and spots of the complementary
biotinylated bovine serum albumin (BSA) on the substrate. Lines
204-207 represent simulations of the structures. The parameters
used for the simulations are given in Table 1 below. The
simulations are based on the effective medium formula by Looyenga
(Physica 31, 401-406, 1965), which has been validated for p++-type
PSi (Squire et al, J Lumin 80, 125-128, 1999):
n.sub.PSi.sup.1/3=(1-p)n.sub.Si.sup.1/3+pn.sub.air.sup.1/3
[0080] The starting parameters of the simulation (layer thickness
and porosity) were taken from the etching program which calculates
current density and etch times for a desired layer thickness and
porosity from calibration curves. The values were then refined to
achieve good agreement between the measured spectrum and the
simulation. For a number of samples the total thickness of the PSi
sample was determined by profilometry to validate the layer
thickness values used in the simulations. In FIG. 2a-d, L=low
porosity (high refractive index) layer, H=high porosity (low
refractive index) layer, S=spacer layer, d=layer thickness,
n=refractive index. The structure of the microcavities is
(LH).sub.7L-S-(LH).sub.9L.
TABLE-US-00001 TABLE 1 layer d/nm n as prepared L 62 2.24 H 91 1.60
S 186 1.60 GaAs L 62 2.25 H 91 1.62 S 187 1.62 silicon dioxide L 62
2.26 H 91 1.61 S 184 1.61 polycarbonate L 62 2.13 H 91 1.62 S 182
1.62
[0081] The reflection spectra 200-203 of the optical cavity are
characterized by sharp `dips` 208-211 in the reflectivity at the
resonant frequency in the Bragg plateaus 212-215 (the regions of
high reflectivity). The position and spectral width of the
resonance is a sensitive measure of the structure and quality of
the cavity. As can be seen in FIG. 2a-d, the cavity resonance is at
approximately the same frequency (wavelength) and has approximately
the same width for all substrate types, indicating that the cavity
is impervious to the substrate.
[0082] As a self-assembly approach, an advantage of the described
embodiments is the possibility of depositing several components
simultaneously without the need to individually align them at the
desired locations on the substrate, as this task is performed by
the biorecognition. Another benefit of using biorecognition to
assemble optical structures in the example embodiments is the
possibility to self-assemble different optical components onto the
same substrate by using different biorecognition pairs. This
concept is demonstrated in FIG. 3b showing the attachment of two
different microcavities 300, 302 with distinct resonant
frequencies, onto different locations of the same substrate 304
which, in this example embodiment, is a polycarbonate film. The
measured reflectivity spectra 306, 308 of the two different
microcavities 300, 302 assembled on the same polycarbonate
substrate 304 as directed by biomolecular interactions are shown in
FIG. 3a. Lines 310, 312 represent simulations of the structures.
FIG. 3b also schematically shows the biorecognition pairs 313, 315
for the respective structures 300, 302 deposited at defined
positions on the substrate 304.
[0083] In this example, at location B the substrate 304 is modified
with avidin 314, whilst at location A the substrate 304 is modified
with biotinylated BSA 316. The two separate free standing
microcavities, B' 300 and A' 302, are modified with biotinylated
BSA 318 and avidin 320, respectively. Biorecognition therefore
dictates that cavity A' 300 assembles at position A, and similarly,
the avidin modified cavity B' 302 binds to the biotinylated
substrate 304 at location B. It was found that cavity B' 302 did
not assemble over spot A or vice versa. Also, there is no need to
align each optical cavity 300, 302 precisely with its respective
receptor spot(s) 314, 316 on the substrate 304. Unbound regions of
the deposited free-standing structure simply break away during the
washing step (compare FIG. 1, Step e).
[0084] In other embodiments, biorecognition is also capable of
self-assembling optical devices from separate components. In one
example, PSi microcavities were assembled from two independent
Bragg mirrors using biorecognition to create the desired resonant
cavities. The steps used are shown in FIG. 4, which shows a
schematic representation of the assembly of microcavities from
parts in one example embodiment. A free-standing Bragg mirror 400
with spacer layer 402 is bound to a substrate Bragg mirror 404 via
biomolecular interactions. The free standing PSI film 406
consisting of the Bragg mirror 400 and the spacer layer 402 is
placed onto the PSI Bragg mirror 404 that was grown on a substrate
410. Biorecognition is used to mate the two parts to form the
cavity 412. The assembly of microcavities was chosen to demonstrate
the robustness and integrity of the biomolecular self-assembly
approach as any non-uniformity in the produced spacer layer
microcavity will result in poor optical characteristics.
[0085] To test the formation of a cavity resonance, the
reflectivity spectra 500, 502 of the structures were measured
before and after assembly of the mirrors, shown in FIGS. 5a and b
respectively. Prior to assembly of the free-standing mirror, the
Bragg plateau of the substrate mirror spans a wavelength range of
550 to 700 nm. The successful assembly of the microcavity on the
substrate is confirmed by the appearance of the pronounced cavity
resonance 504 at 620 nm. As the cavity resonance is particularly
sensitive to the parallelism of the two mirrors and the homogeneity
of the spacer layer, it can be concluded that self-assembly based
on biorecognition in this example embodiment is compatible with
optical manufacturing of subtle devices. The deposited Bragg mirror
consists of seven periods of alternating low and high porosity
layers followed by a high porosity spacer layer. Lines 506, 508
represent simulations of the reflectivity. L=low porosity (high
refractive index) layer, H=high porosity (low refractive index)
layer, S=spacer layer. The parameters used for the simulations are
given in Table 2.
TABLE-US-00002 TABLE 2 Layer d (nm) n Bragg mirror L 62 2.08 H 91
1.63 Microcavity L 62 2.08 H 91 1.60 S 184 1.60
[0086] To further test this capability, several cavities with
spacer layers of different optical thicknesses (which can be
achieved either by varying the thickness or the porosity of the
layer) were fabricated via deposition of a Bragg mirror with
integral spacer layer, and the cavity resonance was always in
agreement with theoretical predictions. FIGS. 6a to c show
reflectivity spectra 600, 602, and 603 of a substrate Bragg mirror
(BM) and different assembled microcavity structures respectively,
assembled on the same substrate as directed by biomolecular
interactions using the approach described above with reference to
FIG. 4. Lines 604, 606, and 607 represent simulations of the
structures. The parameters used for the simulations are given in
Table 3. In FIG. 6, L=low porosity (high refractive index) layer,
H=high porosity (low refractive index) layer, S=spacer layer.
TABLE-US-00003 TABLE 3 layer d/nm n Bragg mirror (BM) L 62 2.15 H
89 1.63 microcavity (MC1) L 62 2.15 H 89 1.58 S 169 1.58
microcavity (MC2) L 62 2.20 H 89 1.57 S 256 1.57
[0087] Further evidence for the uniformity of the assembly of
optical structures is obtained from SEM and profilometry
measurements. The SEM image 700 in FIG. 7 shows the edge 702 of a
1.5 .mu.m thick PSi Bragg mirror film 704 bound to a substrate
mirror 706 via biorecognition. The spacer layer of the microcavity
(etched as an integral part of the free-standing mirror) is
apparent as a distinct layer 708 adjacent to the substrate 706. The
uniformity of the binding between the two components over a large
length scale is also apparent in the profilometry trace 800 shown
in FIG. 8. The adhesion resulting from the multiple biomolecular
interactions between the two optical components was sufficiently
robust that the structures remained intact even after prolonged
sonication in water or ethanol.
[0088] Apart from being able to assemble or form high quality
optical structures, the usefulness of the biomolecular
self-assembly technique in the example embodiments is determined by
the success rate of forming the correct device in the correct
location. FIG. 9 provides details of the success rate of assembling
the final microcavity. When the substrate reflector was modified
with biotinylated BSA, 14 out of 15 avidin-modified lift-off
reflectors correctly assembled into the specific microcavity.
Significantly, when the substrate reflector was modified with
either BSA alone (i.e. no conjugated biotin) or avidin, then no
microcavities were successfully assembled. Hence the specific
biological binding reaction is the condition for device assembly in
such embodiments.
[0089] Using separate components to assemble optical structures has
additional benefits. In the case of optical microcavities, the
method of example embodiments can allow complete flexibility in
choosing the mirrors and the spacer layer. FIG. 10 shows assembly
of microcavities on Si using a sandwich approach: First a spacer
layer 1000 is deposited onto a substrate Bragg mirror 1002, in this
example embodiment using assembly of a free standing spacer layer
1000 via bio recognition or an adhesive coating, followed by
assembling the top Bragg mirror 1004 on the spacer layer 1000 via
bio recognition or an adhesive coating. For example, this technique
would make it possible to build vertical cavity surface emitting
lasers (VCSELs) using PSi mirrors and III-V spacer layers, or III-V
mirrors and Er:glass spacer layer, or insert a sensitized spacer
layer into a cavity. FIGS. 11a-d show the optical properties of the
substrate reflector and the formed microcavities where different
spacer layers, grown as separate PSi thin films with different
porosities and thicknesses, were embedded into the cavity Adhesion
was achieved using proteins deposited onto the PSi spacer layer.
FIG. 11a shows the spectrum 1100 of the underlying (substrate)
Bragg mirror consisting of ten periods of alternating high and low
refractive index layers. FIGS. 10b-d show the spectra 1101-1103 of
sandwich structures with different porosity (refractive index) or
thickness spacer layers as indicated. The free-standing Bragg
mirror deposited onto the spacer layer to complete the microcavity
structure consists of 8 periods of alternating low and high
refractive index layers. Lines 1104-1107 show simulations of the
optical structures. The parameters used for the simulations are
given in Table 4.
TABLE-US-00004 TABLE 4 layer d/nm n a) ##STR00001## L H 68 92 2.15
1.62 b) ##STR00002## L H S1 65 95 250 2.15 1.64 1.69 c) )
##STR00003## L H S2 91 500 2.15 1.62 1.69 d) ##STR00004## L H S3 68
91 242 2.16 1.62 2.06 indicates data missing or illegible when
filed
[0090] In a further embodiment, poly(methyl methacrylate) (PMMA), a
common laser gain medium and lithographic material, was spin-coated
onto a substrate mirror followed by assembling a free-standing
mirror to define the microcavity. It was found that by spin-coating
different thickness polymer layers, the frequency (wavelength) of
the final cavity resonance can be easily tuned. This embodiment
enables the integration of organic materials with (inorganic) high
quality optical components.
[0091] FIG. 12a shows reflectivity spectra 1200, 1202 of a PSi
Bragg mirror before and after deposition of an approximately 500 nm
thick layer of PMMA by spin coating respectively. The positions of
the Bragg plateau and the interference fringes do not shift after
deposition of PMMA, which demonstrates that the polymer did not
enter the pores of the PSi structure, i.e. the properties of the
cavity layer can be adjusted without altering the composition and
optical properties of the Bragg mirror. FIG. 12b shows reflectivity
spectra 1204, 1206, and 1208 of microcavities fabricated by the
approach described above with reference to FIG. 10 with a PMMA
polymer spacer layer (deposited by spin coating) of thicknesses of
100 nm, 300 nm, and 500 nm respectively. The thickness was
determined by the manufacturer spin coating PMMA protocol in the
example embodiments.
[0092] FIG. 13 shows a flow chart 1300 illustrating a method of
component assembly on a substrate according to an example
embodiment. At step 1302, a free-standing component having an
optical characteristic is formed. At step 1304, a pattern of a
first binding species is provided on the substrate or the free
standing component. At step 1306, a bound component is formed on
the substrate through a binding interaction via the first binding
species, wherein the bound component exhibits substantially the
same optical characteristic compared to the free-standing
component.
[0093] The high degree of strength and uniformity imparted with
biorecognition or with the use of adhesive coatings and the
prospect of removing unbound material makes the approach in the
example embodiments amenable to lithographic patterning. For
instance, inkjet printing or soft lithographic stamping of proteins
could define the circuit geography and deposition of silicon
photonic material accomplished by the methods of the example
embodiments. Furthermore, the approach can be extended for any
optical material such that patterning different biomolecules for
mixing different components could provide unprecedented ease and
flexibility in optoelectronic circuit construction especially when
taking into account the wide range of surface functionality that
can be introduced on semiconductors (e.g. via hydrosilylation
chemistry for Si and PSi), metals and polymers. Incorporating the
cavity layer separately was demonstrated using thin PSi layers and
PMMA in example embodiments. Different doping schemes can allow
material to be confined exclusively to the cavity layer, a major
advantage to using PSi for lasing applications. Incorporating
alternative polymeric materials into the resultant photonic
assembly is also possible and can open the door for new composite
materials for diverse applications (e.g. laser gain medium, optical
switches, biosensing at the cavity layer etc.).
[0094] The described embodiments provide methods that utilize
biological recognition as a driving force for assembling photonic
components into more complex architectures on a larger range of
substrates. With the continued need to develop robust and flexible
strategies to incorporate photonic components into complex devices,
this advance expands current capabilities into composite materials.
In conjunction with the evolving landscape of lithographic
techniques and nanofabrication, harnessing the power of nature's
complexity with self-assembling systems in the example embodiments
can become a powerful synergistic tool for technological
advancement in e.g. the photonic industries.
[0095] Current strategies for integrating optical components on a
substrate require wafer-to-wafer transfer or photolithographic
masking and etching to define a precise pattern that physically
holds the optical components. In contrast, in the described
embodiments, registration of optical components can be performed by
spotting a biomolecule solution in a defined location. Importantly,
the biomolecule pattern on the substrate dictates the patterning
such that rinsing removes any non-specifically bound optical
material. Thus the example embodiments allow a simple and flexible
method to spatially array optical components which is amenable to
existing liquid handling techniques, such as inkjet printing or
soft lithographic stamping.
[0096] The described embodiments can provide a platform technology
that allows, inter alia, [0097] integration of any optical material
with any substrate thus eliminating issues of compatibility between
the different materials that are better suited for each type of
optical component. [0098] Simple application of a biological
species in a defined pattern dictating the geography for assembling
the component thus providing a simple method of patterning and
registration.
[0099] By integrating different components on any substrate and
simplifying the registration of optical components on the
substrate, the example embodiments can lead to new and novel
materials and even multiple different materials to be incorporated
into optical devices by using the described biological assembly
approach. This described methods in example embodiments have the
potential to revolutionize the way optical devices and integrated
optical circuits are fabricated and thus can lead to improvements
in current technologies and many novel devices.
[0100] The example embodiments can allow virtually unlimited
resources for fabrication diversity. For instance, different
combinations of the four bases of DNA or RNA for hybridization
assembly, using DNA ligands that bind proteins, called aptamers,
can be fabricated and screened using a process called SELEX,
monoclonal/polyclonal antibody production for many different
antigens, phage display library screening to optimize recognition,
use of combinatorial peptide libraries for the selection of
peptides binding to inorganic substrates, protein:protein
recognition. Thus the choice of assembly pairs can be very large
including interactions such as van der Waals forces, hydrogen
bonding, hydrophobic/hydrophilic, metal coordination,
electrostatics, covalent bonding.
[0101] Application of the biological species in the example
embodiments is predominantly aqueous wet chemistry with mild
conditions, thus avoiding any harsh treatment that may damage
sensitive optical components (i.e. high temperature). The
fabrication can represent a `green` approach. Many techniques can
be used and exist to apply biomolecules to a substrate in
well-defined patterns, including ink jet printing and soft
lithography. In the example embodiments, complementary
biorecognition molecules or thin adhesive coatings drive the
assembly of optical components onto virtually any substrate without
requiring any micromachining. Biorecognition or thin adhesive
coatings can allow previously incompatible materials to be
integrated seamlessly on the same device. The biorecognition layer
or adhesive coating may allow interesting `soft` and `hard`
components to be integrated by themselves or as composites with the
optical materials (i.e. responsive polymers and small molecules,
metals, nanoparticles and objects, redox and photosynthetic
proteins, ionic liquids/liquid crystals, lipid layers, cells,
diatoms, silica and polymer beads etc.)
[0102] Embodiments of the present invention can provide a hybrid
top-down/bottom-up strategy for producing optical structures by
biomolecular assembly of high quality optical materials. Labelling
the optical material with a biological receptor and the substrate
with the complementary ligand (or vice versa) can allow the
assembly of any optical structure on any substrate in a well
defined manner. This can allow previously unrealized components to
be assembled together on the same substrate. No micromachining or
masking for lithography is necessary on the substrate and simple
liquid transfer techniques can define the pattern (circuit
geography). Using a biological assembly approach in the example
embodiments can allow flexibility in substrate choice such that any
planar substrate can be patterned with a biorecognition molecule
for assembling optical structures. Thus, any combination of optical
structures may be integrated on any material.
[0103] Assembling new materials/devices using biomolecule directed
assembly or assembly using adhesive thin films of prefabricated
high quality optical components was demonstrated in example
embodiments. Biomolecule directed assembly of two optical
structures can allow formation of a third optical structure, where
the joining of the two optical structures produces a new optical
characteristic in the resulting structure. Furthermore,
incorporating diverse materials into assemblies with high quality
optical components is possible in different embodiments towards a
range of new optical materials.
INDUSTRIAL APPLICATIONS
[0104] Integrated optics. There is no current strategy that allows
the integration of different optical structures onto the same
substrate material. For example, the integration of III-V light
sources and detectors with Si based photonic crystals, modulators
and/or micro-mirrors, with waveguides and non-linear optical
devices on any substrate material in example embodiments
constitutes a major advance in optoelectronics.
[0105] Optical communications. Biomolecule directed self-assembly
in example embodiments can allow improved and easier alignment of
optical components and/or nanostructured materials on fibre optic
devices.
[0106] New optical devices. The integration of many different
optical components and materials together using biorecognition in
example embodiments can open the door to new functional
architectures and optical devices. For example, vertical cavity
surface emitting lasers (VCSELs) using porous silicon mirrors and
III-V spacer layers, or Er:glass spacer layer. Similarly, VCSEL
type architecture with a bio-sensitized spacer layer to make very
sensitive biosensors, or alternative materials into the cavity
(i.e. responsive polymers and small molecules, metals,
nanoparticles and objects, redox and photosynthetic proteins,
molecular wires, carbon nanotubes, ionic liquids/liquid crystals,
lipid layers, cells, diatoms, silica and polymer beads etc. and
composites of the same) could lead to a host of novel devices, such
as lasers or optical switches.
[0107] Sensors. Forming a biorecognition at the interface that is
sensitive to biological species in example embodiments can enable
increased biosensing sensitivity at the cavity layer in contrast to
previous biosensing work that requires penetration through the
mirrors.
[0108] Lab-on-a-Chip. Advances in microfluidic technologies have
progressed towards realizing the integration of fluid handling,
sensing and detection within a single microscale device.
Embodiments of the present invention can be applied to
lab-on-a-chip technologies (i.e. polycarbonate or other polymeric
channels) as a method to integrate optical materials onto a device
for e.g. sensing and detection.
[0109] Photovoltaics. Existing solar cells can be supplemented with
high quality antireflection layers and/or back reflectors in
embodiments of the present invention.
[0110] Targeted Drug delivery and Medical imaging. Fabricating
assembled microparticles from porous silicon with therapeutics
confined in the spacer layer with a stimuli responsive material in
the embodiments of the present invention. For example, after
reaching the target tissue, external (light) or internal
(enzymatic, pH, etc.) stimuli causes release of the drug.
Engineering the optical properties to be read through tissue
(700-1000 nm) may enable monitoring drug delivery or alternatively,
a method for medical imaging.
[0111] Flat-panel display fabrication, in particular light emitting
diode (LEDs) or light emitting crystal (LCD) displays.
[0112] It will be appreciated by a person skilled in the art that
numerous variations and/or modifications may be made to the present
invention as shown in the specific embodiments.
[0113] For example, it will be appreciated that other optical
characteristics of the free-standing device may be substantially
maintained after assembly, other than the transmission/reflectance
spectra described for the example embodiments, and including, but
not limited to, optically tested characteristics of non-optical
devices for substantially maintaining machining tolerances, such as
optical interference based characterisation for assembly of micro
mechanical or micro electro mechanical systems (MEMS) on a
substrate.
[0114] FIG. 14 shows schematic cross-section drawings illustrating
fabrication of a sensor structure 1400 according to an embodiment
of the present invention. This sensor structure 1400 for detecting
stimuli is composed of a stimuli responsive material 1406 (spacer
material) between two PSi Bragg mirrors 1402 and 1408 such that the
spacer material 1406 defines the position of the cavity resonance.
While two PSi Bragg mirrors 1402 and 1408 are illustrated for the
example embodiment, it will be appreciated that the Bragg mirrors
in different embodiments can be formed from different materials.
Furthermore, different material Bragg mirrors may be used in a
single device on the top and the bottom of the spacer material, for
example to extend the optical Bragg plateau.
[0115] PSi Bragg mirrors 1402 and 1408 are formed by anodizing
crystalline silicon in ethanolic hydrofluoric acid solution with a
step function to yield alternating layers of high and low
refractive index (porosity). It will be appreciated that other
techniques may be used for fabrication of the Bragg mirrors,
including, but not limited to, other electrochemical techniques
with different combinations of electrolyte, doping level and type,
and processing conditions. The PSi surfaces 1402 and 1408 can
either be used `as prepared` or are derivatized. In this example
embodiment, the surfaces 1402 and 1408 are by hydrosilylation of
the functional alkene 10-succinimidyl undecenoate 1502 to stabilize
the material and provide a functional group for further
modification as shown in FIG. 15. Details of further modification
provided by the functional group can be found in K. A. Kilian, T.
Bocking, K. Gaus, M. Gal, J. J. Gooding, Biomaterials 2007, 28,
3055., K. A. Kilian, T. Bocking, K. Gaus, J. J. Gooding, ACS Nano
2007, 1, 355., K. A. Kilian, T. Bocking, K. Gaus, J. King-Lacroix,
M. Gal, J. J. Gooding, Chemical Communications 2007, 1936., K. A.
Kilian, T. Bocking, S. Ilyas, K. Gaus, M. Gal, J. J. Gooding,
Advanced Functional Materials 2007, 17, 2884. and K. A. Kilian, T.
Bocking, L. M. H. Lai, S. Ilyas, K. Gaus, M. Gal, J. J. Gooding,
International Journal of Nanotechnology 2007, 5, 170, the contents
of which are incorporated herein by cross-reference.
[0116] In one example, the steps of fabricating the sensor
structure 1400 are illustrated in FIG. 14 (14a-14d). In FIG. 14a, a
Bragg mirror 1402 is formed on a substrate 1404. In FIG. 14b, the
Bragg mirror 1402 is lifted off the substrate 1404 to form a
free-standing component using a short pulse of high current
density.
[0117] FIG. 14c shows forming a separate Bragg mirror 1408 on a
separate substrate 1410 and coating a thin film of a spacer
material 1406 onto the surface of the Bragg mirror 1408.
[0118] In FIG. 14d, the Bragg mirror 1402 lifted off the substrate
1404 is assembled onto the spacer material 1406 forming the sensor
structure 1400 with the spacer material 1406 between the Bragg
mirror 1402 and the Bragg mirror 1408. The process depicted in FIG.
14 forms a sandwich of polymeric material that functions as a
micro-cavity.
[0119] FIG. 15 shows a schematic cross-sectional drawing
illustrating the process of coating a gelatin spacer material 1502
onto a Bragg mirror surface 1504 according to an embodiment of the
present invention. In FIG. 15a, the Bragg mirror surface 1504 is
derivatized by hydrosilylation of the functional alkene
10-succinimidyl undecenoate 1506 and in FIG. 15b, the gelatin
spacer material 1502 is further coated onto the Bragg mirror
surface 1504. The derivatization process in this example embodiment
prevents the gelatin spacer material 1502 from infiltrating into
the Bragg mirror surface 1504. It is noted that the derivatization
of the Bragg mirror surface 1504 in this example embodiment is
optional, since the unmodified Bragg mirror surface is hydrophobic,
thus already inhibiting easy penetration of the spacer material.
Applying the derivatization process can provide additional surface
protection for the Bragg mirror surface, and may provide increased
functionality of the Bragg mirror surface. Furthermore, it is noted
that the derivatization process may also be applied to the lifted
off ("upper") Bragg mirror 1402 (FIG. 14) in different embodiments,
noting that typically it is less important to passivate the upper
mirror against infiltration because the spacer material, for
example gelatin, is typically allowed to solidify before the upper
mirror is deposited.
[0120] FIGS. 16a-16d shows photographs of different stages in the
process of forming a sensor structure 1600 according to an
embodiment of the present invention. FIG. 16a shows the appearance
of a freshly prepared PSi Bragg mirror 1602 on the substrate 1604,
after anodization wherein the colour of the material (shown as
different shades) is directly related to the etching parameters.
After the electrochemical lift off of the Bragg mirror 1602 from
the substrate 1604, the film of Bragg mirror 1602 is flexible as
shown in FIG. 16b. This film 1602 can be mechanically removed from
the substrate 1604 as shown in FIG. 16c. In FIG. 16d, spin-coating
thin films of polymer results in a uniform coating of the gelatin
(not shown in FIG. 16d) onto the base Bragg mirror 1608 of which
the colour is dependent on the thickness of the layer as indicated
by different shades. Successful adhesion of the lift-off Bragg
mirror 1602 by interaction of the Bragg mirror 1602 (a hydrophobic
PSi film) with the structure (consisting of the base Bragg mirror
1608 and the gelatin) is shown by the appearance of the film 1602
in the lighter shade above the base Bragg mirror 1608. In this
case, complete and high quality adhesion is evident from the
absence of a rumpled appearance to the film 1602. Further, the
quality of adhesion can be readily verified by reflectivity
spectroscopy. It is preferable to achieve a high quality and
complete adhesion as a non-uniform adhesion can result in a Bragg
mirror spectrum without any cavity resonance. In this case, the
scanning electron microscopy of the sensor structure 1600 as shown
in FIG. 16e is consistent with uniform and parallel adhesion as
evidenced by a thin film of gelatin 1606 sandwiched between two PSi
Bragg mirrors 1602 and 1608.
[0121] FIG. 17 shows graphs illustrating the optical reflectance
spectra of a sensor structure containing sandwich microcavities
with PSi Bragg mirrors and gelatin spacer layer according to
different embodiments of the present invention. In these
embodiments, the optical thickness of the gelatin film was adjusted
by using different concentrations of gelatin (FIG. 17a--10 mg/mL,
FIG. 17b--17 mg/mL, FIG. 17c--2.5 mg/mL, FIG. 17d--1.25 mg/mL). The
optical properties of the Bragg mirrors used in the fabrication of
the different structures in FIG. 17a-17d are identical and hence
the Bragg plateau spans the same region of the optical spectrum
from approximately 1730 nm to 690 nm. The pronounced differences in
the resonance peak positions as shown in curves 1702a, 1702b, 1702c
and 1702d were achieved by tuning the optical thickness of the
gelatin spacer layer. In general the optical thickness (nd) of the
spacer layer may be adjusted by altering its refractive index (n),
for example, by changing its composition, or by altering its
thickness (d). In the example in FIG. 17, the optical thickness of
the spacer layer was tuned by adjusting the concentration of the
gelatin solution used in the spin coating step from 1.25 mg/mL up
to a concentration of 10 mg/mL. The approach is validated by the
simulations of the structures using an effective medium model in
which the optical properties of the Bragg mirrors were fixed and
only the optical thickness of the spacer layer was adjusted to fit
the experimental curves 1702a, 1702b, 1702c and 1702d. The spacer
layer optical thickness used in the simulations decreases from FIG.
17a to FIG. 17d since it is expected that the spacer layer optical
thickness would decrease for decreasing gelatin concentrations. The
results of the simulations are shown as curves 1704a, 1704b, 1704c
and 1704d. In these examples, the thickness of the spacer layers
ranges from approximately 100 nm (for the structure prepared with
1.25 mg/mL gelatin solution) to 300 nm (for the structure prepared
with 10 mg/mL gelatin solution) assuming a refractive index of 1.4
for the gelatin layer. The positions of the experimentally measured
cavity resonances in curves 1702a, 1702b, 1702c and 1702d are in
good agreement with those of the simulations in curves 1704a,
1704b, 1704c and 1704d. The good agreement between experimental and
theoretical results demonstrates the high quality of the sensor
structure fabricated by the general approach introduced in FIG.
14.
[0122] The operation of the sensor structure in the example
embodiments can be illustrated by experimental results discussed
below with reference to FIGS. 18-21.
[0123] FIG. 18 illustrates the experimental setup for detecting
protease enzymes using the sensor structure 1800 according to an
embodiment of the present invention. In FIG. 18, a gelatin film
1802 that can be reproducibly cast between the Bragg mirrors 1804
and 1806 at different thicknesses using spin-coating is shown. The
gelatin film 1802 lies on top of the substrate 1812. Application of
protease enzyme 1808 to a thin strip of filter paper 1810 causes
rapid degradation of the gelatin 1802 i.e. proteolysis of the
gelatin thin film 1802. The reaction is directly with the spacer
layer. The material is not entering the two Bragg reflectors. As
discussed above, the mirror surfaces are hydrophobic, thus
inhibiting easy penetration by the spacer material. If the material
was to enter the pores of the mirrors a red shift would be observed
because the average refractive index of the structure has been
increased. Note that the operation of the bio-sensing mechanism
effectively destroys the cavity layer, thus leading to a blue
shift. Therefore if water vapour was entering the mirror pores the
resultant red shift due to air being replaced by water would mask
this blue shift.
[0124] FIG. 19 shows schematic cross-section drawings and spectra
depicting the results obtained using the experimental setup in FIG.
6. In FIG. 19a, the optical reflectance spectrum of a Bragg mirror
1902 is shown whereby the spectrum contains a Bragg plateau 1906.
In FIG. 19b, the optical reflectance spectrum of the assembled
microcavity structure 1900 before proteolysis is shown whereby the
spectrum contains a Bragg plateau 1908 and a cavity resonance 1910.
In FIG. 19c, the optical reflectance spectrum of the structure 1904
(structure 1900 after proteolysis) is shown whereby the cavity
resonance 1910 disappears whereas the position of the Bragg plateau
1912 remains the same. Disappearance of the cavity resonance in
this example is caused by complete digestion of the spacer layer
such that the periodicity of the Bragg mirror is no longer
interrupted. Partial proteolysis would be evident by a shifting of
the position of the cavity resonance to different wavelengths
reflecting the changes in the optical thickness of the spacer layer
(resulting from a change in refractive index or thickness or
both).
[0125] In contrast to previous sensing work, interactions within
the spacer layer 1406 (FIG. 14) in the above example embodiments
affect only the position and magnitude of the resonance without
changing the position of the Bragg plateau (high reflectivity
region). This is because the system in the above example
embodiments is designed in such a way that the analyte of interest,
for example an enzyme, interacts only at the spacer layer 1406
(FIG. 14) and does not infiltrate the nanopores of the PSi
nanoporous structure 1408 by keeping the pore space hydrophobic.
Further details of keeping the pore space hydrophobic in the
nanoporous structure 1408 (FIG. 14) can be found in K. A. Kilian,
T. Bocking, K. Gaus, J. J. Gooding, Angew. Chem. Int. Ed. 2008,
47(14), 2697-2699 the contents of which are incorporated herein by
cross-reference.
[0126] FIG. 20 shows further experimental results using the
experimental setup in FIG. 18 according to an embodiment of the
present invention.
[0127] The Bragg mirror was adhered to the gelatin layer by
allowing the PSi to come into close contact with the gelatin under
ethanol and the resulting sandwich was allowed to dry under a slip
of filter paper in ambient air. After drying, a well-defined cavity
resonance appears central to the Bragg plateau as shown in curve
2002 in FIG. 20a. Next, a small quantity of phosphate buffered
saline was added to the sample by applying a 5 .mu.l drop to a
piece of filter paper adhered to the top mirror. The fluid wicked
up the paper to make contact with the spacer layer and was
incubated for 15 minutes until the paper became dry. Measuring the
spectrum at the same location after buffer addition resulted in a
negligible change to the spectral qualities of the sample as shown
in curve 2004 in FIG. 20a.
[0128] In FIG. 20b, curves 2006 and 2008 respectively illustrate
the optical reflectance spectra before and after the addition of an
enzyme (5 .mu.l of 1 nM subtilisin (5 fmoles)). It can be seen that
this addition resulted in a large shift (32 nm) of the photonic
resonance (curve 2008) compared to the control position (curve
2006) as indicated by arrow 2010.
[0129] FIGS. 20a and b show that the addition of the enzyme has
resulted in a shift of the gelatin resonance position by enzymatic
digestion of the film. This shows that the sensor structure in the
example embodiments works effectively as a sensing device for
stimuli such as the protease enzyme.
[0130] FIG. 21 shows the optical reflectance spectra before and
after proteolysis occurs in a sensor structure according to an
embodiment of the present invention. The optical reflectance
spectra before and after proteolysis are shown in FIGS. 21a and 21b
respectively. In this example, the gelatin layer remaining between
the top and bottom Bragg mirror after proteolysis is negligible (ie
it has essentially been digested) such that proteolysis results in
complete disappearance of the cavity resonance as shown in FIG.
21.
[0131] FIG. 22 shows a plot illustrating the shift in the optical
spectrum after exposure of a sensor structure to water vapour
according to an embodiment of the present invention. After exposing
the gelatin microcavity to water vapour, there is a distinct red
shift (9 nm normalized to the shift in Bragg plateau) in the
position of the cavity resonance as the gelatin swells by
incorporating water molecules within the hydrogel layer. This is
shown in the shift from curve 2202 to curve 2204 in FIG. 22. This
shift is evident visually by a change in the color of the thin film
as it swells. In contrast to organic vapor sensing, the Bragg
plateau remains at approximately the same position due to the
hydrophobic surface disallowing any influx of water molecules.
[0132] In other embodiments, by monitoring the change in the Bragg
plateau, detection of species that do penetrate hydrophobic spaces
could be assessed. For example, exposure to ethanol can cause a
predictable shift of the entire Bragg plateau as the surface
tension of ethanol allows it to penetrate a hydrophobic nanoporous
material. Concurrently monitoring the position of the cavity
resonance that is sensitive to materials that only interact in the
spacer layer may allow simultaneous detection of different species
by separating the spectrum into changes in the cavity resonance or
Bragg plateau. In this way, surface chemistries and spacer layers
that respond to different chemicals and stimuli could be applied to
this device in the example embodiments, allowing multi-analyte
sensing.
[0133] Furthermore, in alternative embodiments the surface
chemistry of the top and bottom optical materials may be tailored
so as to allow flexibility in design. For instance, this can be
done by allowing water or organic solutions to penetrate the porous
silicon via tailored surface chemistry or by providing recognition
elements within or on the top or between sensor structures.
[0134] In other example embodiments, different passive optical
materials such as microcavities, filters, waveguides, etc. can also
be joined together with a wide variety of functional materials such
as photo, thermal and pH responsive polymers and small molecules
(dyes) in polymer matrices, metals, semiconductors, nano and micro
particles and objects, quantum dots, redox and photosynthetic
proteins, viral capsids, self-assembling biomolecules, carbon
nanotubes, buckyballs etc. The joining of many different optical
materials that may work alone or synergistically can convey single
or multiple recognition events.
[0135] FIG. 23 shows a method 2300 of fabricating a sensor
structure according to an example embodiment. In step 2302, a first
Bragg mirror is provided. In step 2304, a second Bragg mirror is
provided and in step 2306, a stimuli responsive material disposed
between the first and second Bragg mirrors is provided. The second
Bragg mirror is assembled on the first Bragg mirror by a binding
interaction via the stimuli responsive material.
[0136] The advantages of the embodiments of the present invention
can include: The detection limit of less than 10 fmoles (i.e. the
least amount of stimuli required to produce a detectable output, in
this case loss of the cavity resonance) using the embodiments of
the present invention is 1000-fold greater than other existing
label-free optical approaches.
[0137] Also, the assay setup in the example embodiments is simple
without any labelling requirements. Only the employment of a simple
light source and detector is necessary.
[0138] This allows the device in the example embodiments to be in a
portable format and be easily used by a simple application of
fluid. The device can also be designed to yield a colour change
visible by the naked eye, allowing it to be more user-friendly.
[0139] In addition, the device in example embodiments can allow a
faster and a more sensitive optical detection of molecules as
compared to prior art devices. In one example, it can detect low
levels of biological species within only 15 minutes. The high speed
and high sensitivity are achieved in the example embodiments as
sensing occurs at the interface between two optical materials, thus
reducing the recognition area to a path on the order of the
wavelength of light. This means that transduction of recognition
and response occurs more rapidly and the sensitivity to the analyte
is increased. Furthermore, fast and sensitive optical detection of
molecules can be achieved In the example embodiments, because the
stimuli responsive material is accessible from the sides, there is
no requirement of analyte diffusion through the Bragg mirror to
reach the stimuli responsive material, which can decrease the
response time compared to existing sensor structures.
[0140] On the other hand, modifying the base layer (Bragg mirror)
with one or more specific chemistry for an analyte of interest,
incorporating a cavity layer that responds to a different
recognition or stimuli, and/or modifying the top layer (Bragg
mirror) with another specific chemistry or chemistries in different
embodiments can allow two or more separate responses that can be
deconvoluted to provide information about multiple interactions
and/or stimulations.
[0141] The device in the example embodiments can be fabricated with
low cost materials. Patterning of the materials is well
established, involves inexpensive materials and is amenable to
self-assembly strategies. In addition, complementary biorecognition
molecules can drive the assembly of optical components onto
virtually any substrate without requiring any micromachining.
[0142] Also, in the example embodiments, optically flat adhesion
using protein-based adhesive can enable many different combinations
of sensor structures and other materials across a surface
(patterning) and vertically to realize novel hybrid materials that
respond in a well-defined way to various chemicals and stimuli. For
example, patterning Bragg mirrors across a surface with a specific
chemistry (covalent, hydrophobic, ionic, H-bonding) can allow
precise deposition of responsive material to form the cavity.
Alternatively, photolithography may allow patterning of a polymeric
(or hybrid) material spatially across an optical material for
subsequent recognition and assembly of another optical
material.
[0143] Applications of the device in the example embodiments
further include the use of the device as a biological sensor,
chemical sensor, temperature, light, pH, voltage or mechanical
sensor or as integrated optics for a Lab-on-a-Chip. When using the
device in the example embodiment as a biological sensor, detecting
biomolecules within the cavity layer between PSi sensor structures
can enable faster detection with enhanced sensitivity without any
requirements for infiltration within nanopores. When the device is
used as a chemical sensor, detection of chemical species can occur
within the PSi crystals or within the cavity layer. Tailoring the
cavity material and the surface chemistry of the PSi to respond to
one or multiple species will enable multiplexed analysis. In
addition, incorporating responsive materials in the cavity will
allow detection of other stimuli when the device in the example
embodiments is used as a temperature, light, pH, voltage or a
mechanical sensor. Furthermore, the responsive materials in the
example embodiments can be integrated into microfluidic circuits
with detectors for lab-on-a-chip type applications.
[0144] It will be appreciated by a person skilled in the art that
numerous variations and/or modifications may be made to the present
invention as shown in the specific embodiments.
[0145] For example, the stimuli responsive material may comprise
one or more of a group consisting of gelatin, extracellular matrix
biopolymers, proteins, oligosaccharides, proteoglycans, recombinant
polypeptides, synthetic polypeptides, nucleic acids, synthetic
co-polymer systems, small molecule and nano-object encapsulated
polymers, pNIPAM, lipids, carbohydrates, cellulose, cells, plant or
animal tissue, polymers of any type, hydrogels, microorganisms,
nanoparticles or nanowires.
[0146] Furthermore, the surface of one or both of the Bragg mirrors
may be derivatized using one or more of a group consisting of
succininide ester, carboxylic acids, Amines, Maleimides, Epoxides,
Azides, Alkynes, alcohols, carbodiimides, aldehydes, diazoniums,
imines, acid chlorides, disulfides, and anhydrides.
[0147] FIG. 24 shows a schematic cross-sectional view of a light
emitting device 2402 according to an example embodiment. The light
emitting device 2402 comprises an upper Bragg mirror 2404 bound on
a lower or substrate Bragg mirror 2406 through a binding
interaction via a light emitting material 2408. The light emitting
material 2408 comprises quantum dots (QDs), in the example
embodiment colloidal QDs 2410. The QDs 2410 are bound within the
light emitting material 2408 via pairs of biorecognition elements,
e.g. 2412, and complimentary species, e.g. 2414. The light emitting
material 2408 in the example embodiment is diffused into respective
interfacial regions 2416, 2418 of the Bragg mirrors 2404, 2406
respectively, such that the interfacial regions 2416, 2418 form a
host for the light emitting material 2408 including the QDs 2410.
An optical cavity 2420 is formed by the adjacent interfacial
regions 2416, 2418.
[0148] In the following, the fabrications steps for the light
emitting device 2402 in an example embodiment will be
described.
[0149] The substrate Bragg mirror 2406 (low porosity 44%/high
porosity 80%) was formed from a p+ type silicon with a top high
porosity layer. The Bragg mirror 2406 was spotted for 5 minutes
with biotinylated BSA, rinsed with PBS, and then spotted for 5
minutes with a solution containing streptavidin conjugated
CdSeTe/PbS colloidal quantum dots, followed by a final rinse.
[0150] The upper Bragg mirror 2404 was fabricated with an inverted
structure on a separate silicon substrate, and lifted off the
silicon substrate. Biotinylated BSA was again applied to the Bragg
mirror 2404, and then the Bragg mirror 2404 was attached, creating
a high porosity cavity or spacer layer (compare interfacial regions
2416, 2418) with the light emitting material, including the QDs, in
the center.
[0151] FIG. 25 shows a scanning electron microscopy (SEM) image of
the fabricated structure in an example embodiment, illustrating the
high porosity cavity or spacer layer 2502 with the light emitting
material, including the QDs, in the center (not resolved in FIG.
25).
[0152] FIG. 26 shows reflectance spectra measured for different
stages of the fabrication of the light emitting device of the
example embodiment. Curve 2602 is the reflectance spectrum obtained
from the bottom or substrate Bragg mirror 2406 (FIG. 24). Curve
2604 shows the reflectance spectrum for the Bragg mirror 2406 (FIG.
24) after spotting with the biotinylated BSA. Curve 2606 shows the
reflectance spectrum of the Bragg mirror 2406 (FIG. 24) after
spotting with the streptavidin conjugated CdSeTe/PbS colloidal
quantum dots. Finally, curve 2608 shows the reflectance spectrum of
the entire light emitting device 2406 (FIG. 24), with the cavity
resonance mode 2610 at about 627.5 nm. The thickness of the high
porosity cavity layer (compare 2502 in FIG. 25) was chosen in the
example embodiment to match the emission wavelength of the quantum
dots at about 625 nm.
[0153] FIG. 27 shows the measured photo luminescence of the light
emitting device for the example embodiment. For the measurement
shown in FIG. 27, the light emitting device was optically pumped
using an argon ion laser with a wavelength of 514.5 nm, at 5 mW.
The high resolution photo luminescence measurement (curve 2702)
shows a strong QD emission from the optical cavity, with a
linewidth of the emission band of about 6.5 nm, which is consistent
with the linewidth of the cavity mode as measured in the
reflectivity spectrum (compare curve 2608 in FIG. 26).
[0154] FIG. 28 shows a comparison of the photoluminescence measured
for the example light emitting device in curve 2802, with a photo
luminescence measurement for the same QDs deposited on silicon
using the same fabrication times, in curve 2804. The intensity for
curve 2804 has been multiplied by 500 in FIG. 28. As can be seen
from FIG. 28, a strong modification of the QD emission by the
optical cavity is observed, with the enhancement in the peak
intensity being of the order of 2000 times. This enhancement is
higher than what would be expected from a cavity with a Q-factor of
about 100 as in the example embodiment of the light emitting
device. To investigate this high enhancement, in a further
experiment a comparison between the photoluminescence of the
example device and the same QDs deposited on the substrate Bragg
mirror was made. In that experiment, the intensity enhancement was
only of the order of five times, which is consistent with what
would be expected for a cavity with a O-factor of about 100. This
experiment suggests that the porous scaffold, i.e. the porous
silicon in the example embodiment, is playing a significant role in
concentrating the QDs, thus significantly contributing to the
emission enhancement.
[0155] FIG. 29 shows a plot 2902 of photoluminescence intensity
versus excitation power from 1 .mu.W to 5 mW, and FIG. 30 shows a
plot 3002 of photoluminescence intensity versus excitation power
from 10 mW to 1 .mu.W, for the example light emitting device. From
FIGS. 29 and 30 it can be seen that a substantially linear trend
over seven orders of magnitude was found, with no observable
evidence of lasing occurring. As will be appreciated by a person
skilled in the art, evidence of a lasing threshold would be
observed by an exponentially increasing region.
[0156] FIG. 31 shows photoluminescence intensity versus incubation
time graphs for different streptavidin-QD incubation times of 10
minutes (curve 3102), 2 hours (3104), and 4 hours (curve 3106).
From FIG. 31, it can be seen that increasing the incubation time
increases the photo luminescence intensity, believed to be due to
an increase in the number of QDs deposited.
[0157] In different embodiments, the optical device may be
optimised by varying the composition of the light emitting
material, including the QDs. For example, a layer by layer approach
with alternately streptavidin and biotin coated QDs to form a
stacked light emitting material structure may be employed to seek
to optimize the performance of the light emitting device.
Alternatively or additionally, different types of QDs may be
incorporated, including incorporating different types of QDs in
different lateral areas within a layer, incorporating different
types of QDs in different layers, or both. In such embodiments,
optical devices for different desired applications can be realised,
e.g. multi-color light emitting devices, light emitting devices in
which one or more types of QDs are optimised for absorption of the
pump energy, while one or more other types of QDs are optimised for
light emission through energy transfer from the QDs optimised for
absorption, or absorption based optical devices including devices
in which different types of QDs are configured in a photo-voltaic
cell arrangement, e.g. in a p-n junction(s).
[0158] FIG. 32 shows a flow chart 3200 illustrating a method of
fabricating a light emitting device according to an example
embodiment. At step 3202, a first Bragg mirror is provided. At step
3204, a second Bragg mirror is provided. At step 3206, a light
emitting material disposed at an interface between the first and
second Bragg mirrors is provided, wherein the second Bragg mirror
is assembled on the first Bragg mirror by a binding interaction via
the light emitting material.
[0159] The example light emitting device described provides a
silicon integrated light emitter, which can have applications in
integrated silicon based optoelectronic devices. The applicant is
not aware of quantum dot doped microcavities formed using Si
integrated optical epitaxial techniques having been reported
before.
[0160] It will be appreciated by a person skilled in the art that
numerous variations and/or modifications may be made to the present
invention as shown in the specific embodiments.
[0161] For example, it will be appreciated that the light emitting
device may be optically or electrically pumped, using different
optical or electrical sources. Furthermore, while II-VI QDs were
used in the example embodiment, it will be appreciated that other
QDs may be used in different embodiments, including III-V QDs.
Furthermore, a gain material may be incorporated into the light
emitting material, to facilitate lasing.
[0162] Also, while a biotinylated BSA and protein avidin pair has
been described, it will be appreciated that other pairs of
biorecognition elements and complimentary species may be used in
different embodiments.
* * * * *